C.Y. Zhao

Designing ultrabroadband absorbers based on Bloch theorem and optical topological transition

In this Letter, we propose a method to design ultrabroadband near-perfect absorbers, consisting of a periodic dielectric-metal multilayer. In the method, the Bloch theorem and optical topological transition (OTT) of iso-frequency surfaces are employed to manipulate the start and end of the near-perfect spectral absorption band, respectively. Moreover, we design and fabricate an ultrabroadband near-perfect absorber utilizing the proposed method. The average absorption of the designed absorber is ∼95% in the focused visible and near-infrared range (0.4-2 μm). This omnidirectional and polarization-independent near-perfect absorber is promising for solar energy harvesting, emissivity control, and thermal imaging.

Design of metasurface polarizers based on two-dimensional cold atomic arrays

Engineering light-matter interaction using cold atomic arrays is one of the central topics in modern optics. Here we have demonstrated the capability of two-dimensional asymmetric cold atomic arrays as microscopic metasurfaces for controlling polarization states of light. The designed linear polarizer can lead to an extinction ratio over 20dB as well as a high transmittance over 0.8 for the permitted polarization at zero detuning. For detuned driving light, changing lattice constants can also achieve high performance linear polarizers. We have also accomplished a circular polarizer by manipulating the phases of transmitted light. A theoretical analysis based on Bloch theorem shows the underlying mechanism for this performance is actually attributed to cooperative effects in periodic lattices. Finally, we discuss in detail the effects of system size, lattice imperfection and nonzero driving light linewidth in practical implementation. The present study paves a way to design extremely miniaturized metasurfaces using cold atoms and other two-level systems, showing great potential in quantum information and quantum metrology sciences as well as the fundamental physics of light-matter interaction.

Study on a Novel Selective Solar Absorber With Surface Ultrathin Metal Film

In this paper, simple selective solar absorbers with three layers are investigated, and their selective absorptivity spectra are quite appropriate for high performance solar absorbers. The simple solar absorber contains top ultrathin tungsten (W) layer, middle silica layers and W substrate. The thickness of silica can determine the location of absorptivity peak while the thickness of top W layer affects the intensity of absorptivity. Considering the total conversion efficiency, optimized thicknesses in solar absorbers are determined by genetic algorithm. This optimized thin film solar absorber keeps high absorptivities when incident direction varies from 0 degree to 60 degree in both TE and TM polarizations. Experiments validate the effectivity of thin film solar absorbers, and the deviation from simulations comes from increscent refractive index and surface non-uniform.Copyright

Light Trapping in Semiconductor Thin Film With Disordered Nanoholes

Finite-difference time-domain method is employed to investigate the optical properties of a semiconductor thin film patterned with circular holes. The presence of hole arrays, although reduces the amount of material usage, can greatly enhance the integrated absorption of the thin film. The optimal square hole lattice can enhance the integrated absorption by a factor of 5.6 over a bare thin film. It is also found that disorderness, including non-uniform radius and random position, can further enhance the absorption efficiency and broaden the absorption spectra. The effects of random position and non-uniform radius are found to be quite different: while absorption spectrum for thin film containing randomly positioned holes is almost broadband, the non-uniform hole radius only slightly broadens the absorption peaks of the periodic structures. The absorption enhancement in the disordered structure is attributed to the increased number of guided resonances in the structure. We also show that with carefully designed hole pattern the overall absorption can be further enhanced.© 2013 ASME